The Claudin polypeptides are a related group of “tetraspan” polypeptides, polypeptides having four membrane-spanning or transmembrane domains, that are associated with cellular tight junctions. Tight junctions, which are also called “zona occludens”, form a regulated, semipermeable barrier in the intercellular spaces within sheets of epithelial or endothelial cells. Inadequate or improperly regulated epithelial or endothelial barrier function contributes to the initiation, maintenance, and exacerbation of inflammation in tissues such as the gut, lungs, etc. Tight junctions also form a “fence” separating the apical and basolateral regions of these cells' membranes, allowing the establishment of different physiological environments on the opposite sides of a cell sheet, such as the different physiological environments required for transport of materials across the intestinal epithelium. It has also been proposed that tight junctions contain aqueous pores, with paracellular transport between the cells of an epithelial or endothelial sheet occurring through these pores. Claudin family polypeptides are expressed in epithelial cells and/or endothelial cells throughout development, with individual members of the Claudin polypeptide family being expressed in different tissues. The physiological functions associated with a particular Claudin polypeptide are related to the functions performed by the particular tissue(s) in which it is expressed.
Common structural features of the Claudin family of polypeptides are the four membrane-spanning (transmembrane) domains, the two extracellular loops formed by the transmembrane domains, and the cytoplasmic tail domain. The cytoplasmic tail domain is thought to be involved in interactions with other tight-junction-associated proteins such as the ZO (zona occludens) family of proteins. These interactive activities of Claudin polypeptides are thought to involve PDZ-domain-containing polypeptides, with a PDZ domain binding to the C-terminal residues of the cytoplasmic tail domain of a Claudin polypeptide; association of PDZ-containing polypeptides may then result in oligomerization of Claudin polypeptides. The extracellular loop domains of Claudin polypeptides may contribute to tight junction formation, which is an important aspect of both the barrier function and the ion transport function of Claudin polypeptides, and/or act as a receptor for viral proteins, enterotoxins, or allergens. The tight junction formation activities of the Claudin polypeptide family are believed to occur through homotypic interactions with the extracellular loops of the same Claudin polypeptide expressed on neighboring epithelial or endothelial cells, or heterotypic interactions with the extracellular loops of other Claudin family members or other non-Claudin polypeptides. In addition, there is evidence that the biological effects of Claudin polypeptides involve a requirement for Claudin polypeptides, and particularly their most N-terminal extracellular domain, in the processing of matrix metalloproteinases to their active form (Miyamori et al., 2001, J Biol Chem 276: 2804–28211). Because of their roles in tight junction formation, epithelial and endothelial barrier function, ion transport, and viral protein, enterotoxin, or allergen binding, Claudin polypeptides are associated with conditions involving unregulated or improperly regulated transport across the epithelium or endothelium such as inflammation, asthma, allergy, metastasis of cancer cells, and ion transport disorders such as magnesium transport defects in the kidney. In addition, because a Claudin polypeptide expressed in neural cells has been shown to be required for formation of the myelin sheath in oligodendrocytes, Claudin polypeptides are associated with demyelination conditions such as multiple sclerosis (MS), autoimmune encephalomyelitis, optic neuritis, progressive multifocal leukoencephalopathy (PML), etc.
Characteristics and activities of the Claudin polypeptide family are described further in the following references: Fujitaab K et al., 2000, Clostridium perfringens enterotoxin binds to the second extracellular loop of claudin-3, a tight junction integral membrane protein, FEBS Lett. 476: 258–261; Kinugasa T et al., 2000, Claudins regulate the intestinal barrier in response to immune mediators, Gastroenterology 118: 1001–1011; Tsukita S and Furuse M, 2000, Pores in the wall: claudins constitute tight junction strands containing aqueous pores, J Cell Biol. 149: 13–16; Bronstein J M et al., 2000, Involvement of OSP/claudin-11 in oligodendrocyte membrane interactions: role in biology and disease, J Neurosci Res. 59: 706–711; Itoh M et al., 1999, Direct binding of three tight junction-associated MAGUKs, ZO-1, ZO-2, and ZO-3, with the COOH termini of claudins, J Cell Biol. 147: 1351–1363; Furuse M et al., 1999, Manner of interaction of heterogeneous claudin species within and between tight junction strands, J Cell Biol. 147: 891–903; Morita K et al., 1999, Endothelial claudin: claudin-5/TMVCF constitutes tight junction strands in endothelial cells, J Cell Biol. 147: 185–194; Kubota K et al., 1999, Ca(2+)-independent cell-adhesion activity of claudins, a family of integral membrane proteins localized at tight junctions, Curr Biol. 9: 1035–1038; Wan H et al., 1999, Der p 1 facilitates transepithelial allergen delivery by disruption of tight junctions, J Clin Invest 104: 123–133; Simon D B et al., 1999, Paracellin-1, a renal tight junction protein required for paracellular Mg2+ resorption, Science 285: 103–106; Morita K et al., 1999, Claudin multigene family encoding four-transmembrane domain protein components of tight junction strands, Proc Natl Acad Sci USA. 96: 511–516; Furuse M et al., 1998, A single gene product, claudin-1 or -2, reconstitutes tight junction strands and recruits occludin in fibroblasts, J Cell Biol. 143: 391–401; Furuse M et al., 1998, Claudin-1 and -2: novel integral membrane proteins localizing at tight junctions with no sequence similarity to occludin, J Cell Biol. 141: 1539–1550; all of which are incorporated by reference herein.
In order to develop more effective treatments for conditions involving disruption of epithelial or endothelial barrier function or unregulated transport across the epithelium or endothelium, such as inflammatory bowel disease, or involving demyelination, such as multiple sclerosis, information is needed about previously unidentified members of the Claudin polypeptide family, so that the characteristics and activities of such new Claudin family members can be ascertained. In particular, there is a need for characterization of previously unidentified human Claudin polypeptides.